1. Field of the Invention
The present invention relates to a fluorescent lamp driving apparatus and, more particularly, to a circuit protection technique for the apparatus.
2. Description of the Related Art
In recent years, a thinner and larger-sized liquid crystal television have been widely provided in place of a CRT television. In a liquid crystal television, a plurality of cold cathode fluorescent lamps (hereinbelow, called CCFLs) are disposed in the back surface of a liquid crystal panel on which a video image is displayed, and the CCFLs are allowed to emit light as a backlight.
To drive the CCFLs, for example, an inverter (DC/AC converter) for boosting a direct current (DC) voltage of about 12V and outputting the resultant voltage as an alternating current (AC) voltage is used. The inverter converts an electric current flowing in the CCFLs to a voltage, feeds back the voltage to a control circuit and, on the basis of the feedback voltage, controls the on/off state of a switching device. For example, Patent Document 1 discloses a technique of driving a CCFL by such an inverter.
[Patent Document] Japanese Patent Application Laid-open No. 2003-323994
To suppress increase in the number of CCFLs accompanying enlargement of the size of the liquid crystal panel, a U-shaped CCFL is employed. To drive the U-shaped CCFL, it is necessary to use two inverters to apply AC voltages of reversed phases across the U-shaped CCFL. Such a U-shaped CCFL drive circuit converts secondary current of transformers of two inverters to voltage, feeds back the voltage and, on the basis of the feedback voltage, controls light emission of the CCFL.
Such a drive circuit is provided with an abnormality detection circuit for detecting an abnormal state in which a CCFL fails or connection failure occurs and, as necessary, performing circuit protection. For example, the abnormality detection circuit monitors the feedback voltage, determines a circuit abnormality when the feedback voltage becomes below a predetermined threshold voltage, and performs circuit protection.
The manufacturer of a liquid crystal television conducts a test for detecting a failure in a U-shaped CCFL, a contact failure, or the like before shipment of a television. In the test, in some cases, the user wishes to conduct a test to see whether a predetermined high voltage is applied across the U-shaped CCFL or not in a state where the U-shaped CCFL emits light. There is also a case that a similar test is desired to be conducted at the time of failure maintenance also after shipment.
Since a high voltage exceeding 1,000V is applied to the U-shaped CCFL, a high-voltage probe is used for testing the voltage. The high-voltage probe has large capacity. Consequently, in the case of testing each of the ends of the CCFL, the feedback voltage also fluctuates. Therefore, in the case of determining whether the CCFL is normally driven or not on the basis of the voltage fed back to a control circuit, it is difficult to determine whether the feedback voltage fluctuates due to a failure or by the high-voltage probe.